One of the most common types of containers used for heating food in an oven is the aluminum pan, or other such containers made of a thin sheet of aluminum or aluminum foil. Such aluminum containers may be preformed into a specific shape, thus avoiding the labor required for assembly of the shaped article, and can be easily stacked, thus allowing for easy storage and dispensing therefrom for use. Furthermore, such aluminum containers are resistant to penetration by grease or oil, or by water moisture, and permit good browning of baked goods that are baked therein. An example of these sorts of aluminum containers is disclosed in U.S. Pat. No. 3,494,896 to Smith. Though strong and readily preformable, aluminum containers generally cannot be used in microwave cooking. Consequently, efforts have been made to develop plastic containers and plastic-coated paper structures that can withstand heating in either conventional or microwave ovens--i.e., that are "dual-ovenable." To be "ovenable" for purposes of a conventional oven, a structure should be able to withstand temperatures of up to at least 300.degree. F. and, preferably, about 400.degree. F. or so; to be "ovenable" for purposes of a microwave oven, the structure should be strong at boiling water temperatures and should not reflect microwaves and thereby cause arcing or damage the oven's microwave generation. Furthermore, concurrent with such efforts directed at developing dual-ovenable containers, there has been interest in achieving this result without increasing the cost of the container, or preferable by decreasing such cost.
One development in this area has been the use of extruded films of polyethylene terephthalate (PET), or polybutylene terephthalate (PBT), adhered to a paper material for oven-heatable trays. For example, U.S. Pat. No. 4,900,594 to Quick et al. discloses an ovenable tray made with a bi-axially oriented film of PET, such as MYLAR.TM.. The PET film is adhered to a paperboard substrate by means of an adhesive. A shaped tray is formed from the PET-paperboard laminate by pressure-forming procedures such as that described in U.S. Pat. No. 4,026,458 to Morris et al. However, such PET-paperboard laminates are generally not biodegradable, or not readily so, and are not readily gluable to form trays simply by folding the corners in appropriate configuration and applying an appropriate glue or adhesive. Rather, as noted above, ovenable trays are formed from the PET-paperboard laminate generally by pressure-forming the laminate into the desired shape.
Another approach has been to form a laminate by extruding layers of polymethylpentene, tie resins, and barrier resins onto a paper substrate, as is disclosed in U.S. Pat. No. 5,002,833 to Kinsey, Jr. et al. The resulting product is said to have a high degree of adhesion between the paper substrate and the polymethylpentene food contact layer, and is said to be capable of being utilized for forming pressed or locked corner food trays which can be subjected to oven cooking temperatures. The laminate may be made by co-extruding, directly onto a paper substrate, a three-layer sandwich comprised of a barrier resin (such as polyamide resins, copolyamide resins such as nylon resins, polyester resins, or copolyester resins), a tie resin (such as chemically modified graft copolymers of methylpentene), and a release layer of polymethylpentene (the food contacting layer). The structure so made makes up a three layer coextrusion on a paper substrate. From a manufacturing standpoint, however, the extrusion or coextrusion process is more difficult and costly than a simple coating process and requires appropriate extruding equipment.
Yet another approach has been to coat a mixture of polyvinyl alcohol ("PVA") and a chrome-fatty acid complex known as "Quilon.RTM." onto a non-porous paper substrate such as pan liner paper. Alternatively, the pan liner paper may be coated with PVA followed by a coating of Quilon.RTM.. The pan liner paper so coated may be added to a conventional baking tray to provide release of food from the tray. However, such pan liner paper lacks the dimensional stability and strength to be used to form an ovenable tray by itself. Furthermore, such pan liner paper provides poor food-browning and is expensive relative to less costly, more porous grades of paper, such as Kraft paper or newsprint paper.
It is therefore desirable to produce a sheet material simply and economically that has the strength and dimensional stability to function as a cooking or baking tray, that may be used in both conventional and microwave ovens (i.e., that may be "dual-ovenable"), that is resistant to penetration by water moisture and by grease and oil (particularly at elevated temperatures), and that is readily formable and gluable to form a tray or other shaped article. Furthermore, it is desirable that such a sheet material allow for superior browning of baked goods and good release of items that may be used in the tray. It is further desirable that such sheet material have an aesthetically pleasing appearance.